1. Field of the Invention
One embodiment of the present invention relates to a photocathode that emits photoelectrons in response to incidence of light, and an electron tube and a photomultiplier included with such a photocathode.
2. Related Background Art
A photocathode is, as described in, for example, U.S. Pat. No. 3,254,253 and Japanese Published Examined Patent Application No. H05-52444, a device that emits electrons (photoelectrons) generated in response to an incident light. Such a photocathode is favorably applied to an electron tube such as a photomultiplier tube. In addition, the photocathode can be of two types: a transmission type and a reflection type, according to the difference in supporting substrate materials to be applied thereto.
In a transmissive photocathode, a photoelectron emission layer is formed on a supporting substrate made of a material that transmits an incident light, and a part of the transparent vessel of a photomultiplier tube or the like functions as the supporting substrate. In this case, when an incident light that has transmitted through the supporting substrate reaches the photoelectron emission layer, photoelectrons are generated within the photoelectron emission layer in response to the reached incident light. As a result of an electric field for extracting photoelectrons being formed on the side opposite to the supporting substrate in relation to the photoelectron emission layer, the photoelectrons generated in the photoelectron emission layer are emitted toward a direction coincident with a traveling direction of the incident light.
On the other hand, in a reflective photocathode, a photoelectron emission layer is formed on a supporting substrate made of a material that blocks an incident light, and the supporting substrate is arranged inside of a transparent vessel of a photomultiplier tube. In this case, the supporting substrate functions as a reinforcing member that supports the photoelectron emission layer, and an incident light directly reaches the photoelectron emission layer while avoiding the supporting substrate. Within the photoelectron emission layer, photoelectrons are generated in response to the reached incident light. The photoelectrons generated in the photoelectron emission layer are, as a result of an electric field for extracting photoelectrons being formed on the side opposite to the supporting substrate in relation to the photoelectron emission layer, emitted to the side from which the incident light has traveled in relation to the supporting substrate.